Linux Command Reference

This is a reference guide for commonly used Linux commands. The following commands are available in most Linux distributions and can be accessed from the command line interface (CLI).

Table of Contents

• A
• B
• C
• D
• E
• F
• G
• H
• I
• J
• K
• L
• M
• N
• O
• P
• Q
• R
• S
• T
• U
• V
• W
• X
• Y
• Z

Commands

A

• alias - Create an alias for a command.
• apropos - Search the manual page names and descriptions.
• awk - Pattern scanning and processing language.

B

• basename - Strip directory and suffix from a file path.
• bg - Run a job in the background.
• break - Exit from a loop.
• builtin - Run a shell builtin command.

C

• cal - Display a calendar.
• cat - Concatenate and display files.
• cd - Change the current directory.
• chmod - Change file mode bits/permissions.
• chown - Change file ownership.
• chroot - Run a command with a different root directory.
• cksum - Print CRC checksum and byte counts.
• clear - Clear the terminal screen.
• cmp - Compare two files byte by byte.
• comm - Compare two sorted files line by line.
• command - Run a command bypassing shell functions.
• continue - Resume the next iteration of a loop.
• cp - Copy files and directories.
• cron - Daemon to execute scheduled commands.
• crontab - Schedule a command to run at a specific time.
• csplit - Split files based on context.
• cut - Remove sections from each line of files.

D

• date - Print or set the system date and time.
• dd - Convert and copy a file.
• df - Display disk space usage.
• diff - Compare files line by line.
• diff3 - Show differences among three files.
• dirs - Display the list of currently remembered directories.
• disown - Remove jobs from current shell.

E

• echo - Print arguments to the standard output.
• egrep - Search files for a pattern using extended regular expressions.
• elif - Conditional statement in a shell script.
• else - Conditional statement in a shell script.
• env - Display or set environment variables.
• esac - Conditional statement in a shell script.
• eval - Evaluate several commands/arguments.
• exec - Replace the current process with a new process.

F

• false - Do nothing, unsuccessfully.
• fc - Fix command line.
• fg - Run a job in the foreground.
• file - Determine file type.
• find - Search files and directories.
• fmt - Reformat paragraph text.
• for - Loop command.

G

• gawk - Pattern scanning and processing language.
• getopts - Parse positional parameters.
• grep - Search files for a pattern.
• groups - Print the group names of the current user.

H

• hash - Remember the full pathnames of commands.
• head - Output the first part of files.
• help - Display help information.
• history - Command history.
• hostname - Print or set the system's host name.

I

• id - Print user and group information.
• if - Conditional statement in a shell script.
• in - Loop command.
• info - Help information.
• install - Copy files and set attributes.
• jobs - List active jobs.

J

• jobs - List active jobs.
• join - Join lines of files.

K

• kill - Terminate a process.

L

• less - Display file contents.
• let - Evaluate arithmetic expressions.
• ln - Create links.
• local - Define local variables.
• locate - Find files by name.
• logname - Print the current login name.

M

• make - GNU make utility to maintain groups of programs.
• man - Display manual page.
• mkdir - Create directories.
• mkfifo - Make FIFOs (named pipes).
• more - Display file contents.
• mv - Move/rename files or directories.

N

• nice - Set the priority of a command.
• nohup - Run a command immune to hangups.
• notify-send - Send desktop notifications.
• now - Print the current date and time.

O

• open - Open a file or directory.
• op - An application to handle cryptographic operations.

P

• passwd - Change user password.
• paste - Merge lines of files.
• ping - Send ICMP ECHO_REQUEST to network hosts.
• popd - Change the current directory to the previous directory.
• printenv - Print environment variables.
• printf - Format and print data.
• ps - Report a snapshot of the current processes.
• pushd - Change the current directory to a specified directory.

Q

• quota - Display disk usage and limits.

R

• read - Read a line from standard input.
• readonly - Mark variables or functions as read-only.
• reboot - Reboot the system.
• rename - Rename files.
• return - Exit a function.
• rev - Reverse lines of a file.
• rm - Remove files or directories.
• rmdir - Remove empty directories.
• rsync - Remote file copy (Synchronization).
• run-parts - Run scripts or programs in a directory.

S

• screen - Terminal multiplexer.
• sed - Stream editor for filtering and transforming text.
• select - Generate a menu in a shell script.
• seq - Print numeric sequences.
• set - Set shell options.
• shift - Shift positional parameters.
• shopt - Shell options.
• shutdown - Shutdown or restart the system.
• sleep - Delay for a specified time.
• sort - Sort lines of text files.
• source - Execute commands from a file.
• split - Split a file into pieces.
• ssh - Secure Shell client.
• su - Substitute user identity.
• sudo - Execute a command as another user.
• sum - Print checksum and block counts.
• suspend - Suspend the execution of the current shell.
• symlink - Create a symbolic link to a file.

T

• tail - Output the last part of files.
• tar - Manipulate archive files.
• tee - Read from standard input and write to standard output and files.
• test - Evaluate a conditional expression.
• time - Measure program execution time.
• times - Print the accumulated user and system times.
• touch - Change file timestamps.
• tr - Translate or delete characters.
• trap - Run a command when a signal is set.
• true - Do nothing, successfully.
• tty - Print the file name of the terminal connected to standard input.
• type - Describe a command.

U

• ulimit - Set or report file size limit.
• umask - Set the file mode creation mask.
• unalias - Remove an alias.
• uname - Print system information.
• uniq - Uniquify files.
• unset - Remove variable or function names.
• until - Loop command.

V

• vi - Text editor.
• vmstat - Report virtual memory statistics.

W

• wait - Wait for a process to complete.
• wc - Print newline, word, and byte counts.
• while - Loop command.
• who - Print who is currently logged in.
• whoami - Print the current user.

X

• xargs - Execute utility, passing constructed argument lists.
• xdg-open - Open a file or URL in the user's preferred application.
• xxd - Make a hexdump or do the reverse.

Y

• yes - Output a string repeatedly.

Z

• zcat - Concatenate and display compressed files.

🐚 Bash Cheat Sheet

This cheat sheet helps you navigate, automate, and debug efficiently in Bash. It covers file operations, permissions, variables, regex, flow control, and more.

Useful for:

• Linux/Mac users
• System administrators
• Security professionals
• Students (LPIC, OSCP, etc.)

📂 File Test Operators

Check file types and permissions in scripts.

Example:

if [ -f "/etc/passwd" ]; then
  echo "File exists"
fi

🔢 Integer Comparisons

Example:

a=5; b=10
if [ "$a" -lt "$b" ]; then
  echo "a is less than b"
fi

🔤 String Comparisons

Example:

str="hello"
if [ -n "$str" ]; then
  echo "String is not empty"
fi

🔄 Compound Operators

Example:

if [ -r file.txt -a -w file.txt ]; then
  echo "Readable and writable"
fi

⚙️ Job Control

Example:

sleep 100 &
jobs
fg %1

🔀 List Constructs

Example:

mkdir test && cd test

🚪 Exit Codes

Example:

echo "test"
echo $?   # prints 0 if successful

📡 Signals & Shortcuts

📜 Permissions

Example:

chmod 755 script.sh   # rwxr-xr-x

✂️ String Manipulation

Example:

text="Hello World"
echo ${#text}       # 11
echo ${text:0:5}    # Hello

📌 Command Parameters

Example:

echo "Script name: $0"
echo "First arg: $1"

🔁 History Expansion

🎭 Globbing vs Regex

• Globbing (filenames):

  • * → any string
  • ? → single char
  • [a-z] → range

• Regex (text matching):

  • . → any char
  • * → zero or more
  • + → one or more
  • ^ → start of line
  • $ → end of line

Example (globbing):

ls *.txt

Example (regex with grep):

grep "^[0-9]" file.txt

🔄 Flow Control

# If / Else
if [ "$USER" == "akib" ]; then
  echo "Welcome Akib"
else
  echo "Access Denied"
fi

# For loop
for i in {1..5}; do
  echo "Number $i"
done

# While loop
count=1
while [ $count -le 3 ]; do
  echo "Count $count"
  ((count++))
done

✅ Final Notes

• Use man bash for full docs
• Prefer [[ ... ]] over [ ... ] for conditionals
• Practice with small scripts before automation